1. Technology Field
The present invention generally relates to x-ray tube devices and other filament-containing devices.
2. The Related Technology
X-ray generating devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. Such equipment is commonly employed in areas such as medical diagnostic examination, therapeutic radiology, semiconductor fabrication, and materials analysis.
Regardless of the applications in which they are employed, most x-ray generating devices operate in a similar fashion. X-rays are produced in such devices when electrons are emitted, accelerated, and then impinged upon a material of a particular composition. This process typically takes place within an x-ray tube located in the x-ray generating device. The x-ray tube generally comprises a vacuum enclosure that contains a cathode and an anode. The cathode typically includes a filament structure for emitting electrons that are then received by the anode.
The vacuum enclosure may be composed of metal such as copper, glass, ceramic, or a combination thereof, and is typically disposed within an outer housing. At least a portion of the outer housing might be covered with a shielding layer (composed of, for example, lead or similar x-ray attenuating material) for preventing the escape of x-rays produced within the vacuum enclosure. In addition a cooling medium, such as a dielectric oil or similar coolant, can be disposed in the volume existing between the outer housing and the vacuum enclosure in order to dissipate heat from the surface of the vacuum enclosure. Depending on the configuration, heat can be removed from the coolant by circulating it to an external heat exchanger via a pump and fluid conduits.
In operation, an electric current is supplied to the cathode filament, causing it to emit a stream of electrons by virtue of a process known as thermionic emission. An electric potential is established between the cathode and anode, which causes the electron stream to gain kinetic energy and accelerate toward a target surface disposed on the anode. Upon impingement at the target surface, some of the resulting kinetic energy in converted to electromagnetic radiation of very high frequency, i.e., x-rays.
The specific frequency of the x-rays produced depends at least partially on the type of material used to form the anode target surface. Target surface materials having high atomic numbers (“Z numbers”), such as tungsten or tungsten rhenium, might be employed, although depending on the application, other materials could also be used. The resulting x-rays can be collimated so that they exit the x-ray device through predetermined regions of the vacuum enclosure and outer housing for entry into the x-ray subject, such as a medical patient.
One challenge encountered with the operation of x-ray tubes relates to the speed with which the stream of electrons produced by the filament of the cathode can be turned on and off, commonly referred to as “switching time.” Though advantageous for accurately controlling the electron stream and hence the production of x-rays, it has been traditionally difficult to achieve relatively fast filament switching times due to a number of factors, most prevalently, the thermal response—also referred to herein as the “thermal time constant”—of the filament. Briefly, the thermal time constant is a measure of the time required for the filament to cool to a predetermined temperature. The thermal time constant is directly related to the “time constant,” or measure of time required for the filament to reduce electron emission to a predetermined level. As can be determined from the above, the time constant and switching time of the filament are closely related. Thus, a relatively short time constant corresponds to a desirable fast switching time.
The current design of known filaments does not easily provide for the reduction of switching times. One approach involves the inclusion of a third filament electrode, commonly called a grid, for use in modulating the electron beam emission. While acceptably lowering filament switching times, grids nevertheless carry with them some undesirable consequences. Apart from the extra grid lead and power supply needed to power it, one chief consequence of grid use is the increased risk of electrical arcing from tube structures to the grid itself. This can be particularly acute in tubes utilizing high voltages, and can result in damage to the tube.
Other attempts to acceptably switch and modulate the emitted electron beam, also referred to herein as “beam current,” include the heating of a low thermal mass emitter by an electron beam, or modulation of the electron beam by modulating the electric potential imparted to the anode. However, these options also suffer from a relative increase of the risk for arcing within the tube.